We recently published a clinical study to assess the therapeutic potential of recombinant human CD40 ligand in patients with XHM. We designed a one-center investigator-initiated Phase I/II clinical trial (protocol 00-I-0006) to assess safety of recombinant CD40L administration, and determine if it could restore the core immunologic defects of patients with XHM. Five patients were enrolled, and were treated on a dose escalation schedule for up to one year. We showed that administration of recombinant CD40 ligand is safe and can reconstitute deficient immune responses. Specifically, patients T cells demonstrate for the first time a capacity to synthesize IFN-&#947;and TNF-&#945;when stimulated with anti- CD3, or SEB, or SEA. Studies of cytokine production by intracellular staining demonstrated that recombinant CD40L was able to prime both the CD4 and CD8 T cell populations. In addition, all patients developed positive delayed type hypersensitivity reactions to candida, and KLH, and one patient to mumps antigen. Patients on therapy also demonstrated the development of new adenopathy, improvement of primary follicle formation, expansion of both B and T cell populations in the lymphnodes, and the development of follicular dendritic cells. However, germinal center formation and immunoglobulin class switch recombination (CSR) in B cells was lacking. Further improvements in B cell terminal differentiation may require higher doses or a different class of drugs that offer greater half-life. Stimulated by these observations, we treated two XHM patients who had failed standard antimicrobial therapy for biliary cryptosporidiosis with CP-870,893, a human IgG2 antibody that is agonistic for CD40 and has shown therapeutic efficacy against pancreatic carcinoma. CP-870,893 effectively activated B cells and APCs in vitro, restoring class switch recombination in XHM B cells and inducing TNF-&#945;and IL-12 secretion by XHM MoDCs. CP-870,893 also showed significant activity in vivo, causing decreases of leukocytes from the peripheral blood post-infusion. However, despite promising preclinical data, the patients did not experience significant immune reconstitution and the treatment was not successful in eradicating biliary cryptosporidiosis. We found that the CD40 receptor is rapidly internalized following binding with CP-870,893, providing a plausible mechanism for the limited capacity of CP-870,893 to mediate immune reconstitution in XHM. This study demonstrates that CP-870,893 suppressed oocysts shedding in XHM patients with biliary cryptosporidiosis. The continued study of CD40 agonists in XHM is warranted.